Photocell control system for camera diaphragms



G. G. BRUCK July 13, 1954 PHOTOCELL CONTROL SYSTEM FOR CAMERA DIAPHRAGMS 3 Sheets-Sheet 1 Filed May 24, 1950 \wm A INVENTOR GEORGE G. BRUCK July 13, 1954 G. G. BRUCK 2,683,402

PHOTOCELL. CONTROL SYSTEM FOR CAMERA DIAPHRAGMS Filed May 24, 1950 v 3 Sheets-Sheet 2 I INVENTOR GEORGE G. BRUCK ATTORNEY BIAS-20v `July 13, 1954 PHOTOCELL CONTROL SYSTEM FOR CAMERA DIAPHRAGMS Filed May 24, 1950 G. G. BRUCK 3 Sheets-Sheet 3 FROM POINT :II

s a T 3 o,

f Mm f Y o 5 Nw l- 1\ LIM- wn co INVENTOR GEORGE G. BRUCK @Lwwv ATTORNEY Patented July 13, 1954 PHOTOCELL CIONTROL SYSTEM FOR CAMERA DIAPHRAGIJS George G. Bruck, Central Islip,

N. Y., assigner to Specialties, Inc., Syosset, N. Y., a corporation of New York Application May 24, 1950, Serial No. 163,977

Claims. 1

This invention relates toautomatic electric control systems for automatic exposure control of photographic apparatus subjected to variation of conditions of illumination.

In the art of photography it is often of great benefit to relieve the operator of the necessity of having any concern with light conditions and changes thereof and to provide means to automatically compensate or adjust the camera to preserve a preselected illumination value at the lm or other photosensitive surface. One illustrative use thereof is in the lart of aerial photography and hereinafter such art will be used for purposes of `explanation and illustration but without limiting the applications and uses of the present invention in any way.

Particularly in the art of aerial photography, it is often desired that variations of the illumination of the subject being photographed, such as terrain over which the aircraft is flying, shall be compensated for without needing any attention from the pilot of other operating personnel of the aircraft.

Furthermore, with modern high speed iiight of aircraft, large changes of the illumination of the terrain being photographed by airborne cameras may occur much toorapidly for even a, highly experienced photographer to compensate for the changes in the short/time involved, so that the resultant photographs will be of widely different densities.

As an example, aerial cameras of the strip type, which are adapted to take continuous or strip photographs of the terrain over which the aircraft travels, require adequate and efficient means for maintaining the light intensity falling upon the nlm in the camera at a predetermined value in order that the nally produced photographs can give an accurate representation of-the terrain photographed, Without areas `oflighter or dal' ier density.

The present kinvention has for a specific object the kprovision of completely automatic means for maintaining the light falling upon a Anlm within an aerial camera substantially at a predetermined value, irrespective of the variations of light conditions on the terrain over which the camera is flown for photographic purposes.

A lfurther objectk of the inventionis to prointensity o1" vide automatic means for compensating forvariations in intensity or illumination for use with a camera such as an aerial camera.

Generally, the present invention, `provides an automatic control system for compensatingfor changes of light intensity on an object to be 2 photographed and adjusting the camera characteristics to suit such changes and maintain a preselected light intensity at ilm orother photosensitive surface inthe camera having as its essential elements two opposednetworks one of which includes a photocell and the second of which is a reference network, whereby the cutof-balance oi the two networks can be used to modify and control the amount of light passing through the camera lens.

Usually, an iris is used in a camera to vary the effective area of the lens and thus control the mount of light received by the nlm and hereinafter references to control of iris aperture for this purpose will be made as illustrative of a means conventionally employed to adjust .and control the light intensity at the film.

In the present invention, means provided to manually or automatically set or select shutter speeds to suit conditions to be encountered or changed conditions observed, this preferably comprising changing the resistance of the reference network relative to the photocell network, to thereby alter the point of balance.

Although control of illumination of the film by variation of lens aperture, namely iris adjustment is normally undesirable in photographic work due to variations of depth of field caused thereby, the present invention is particularly adapted for aerial 'photography where under nearly all conditions subjects being photographed are at infinity and therefore depth of field is of no consequence.

Generally, in practising the present invention, an electric motor drives the usual iris of an aerial camera and also drives an iris in front or a photocell of the barrier layer type. Whenever light intensity at the camera Vnlm is correct the motor and irises are motionless.

When a change of light intensity occurathis is observed by theV photocell and causes asignal to be sent to the signal network to indicate' that a new iris setting is ynow required in order to maintain the light intensity at the film 'at the pre-set value. Photocell information is matched against information received from the reference circuit, which Vinformation lis pre-set and Aixed for each equivalent shutter speed. When the .in-- formation yof the two circuits does-not. match, a comparator or electronicV balance system puts out asignal which when vamplified drives the motor in accordance with the polarity of the signal, to open or close the irises.

It is one unique feature ofthe present invention that the-photocell is used with reverse -voltage applied thereto. A barrier layer photocell is a semi-conductor and will conduct in one direction while in the opposite direction it is practically a non-conductor, provided it is not illuminated. However, when illuminated, it will conduct a small amount of current in this opposite direction. If constant voltage is applied in this opposite direction, the current nowing will be substantially proportional to light intensity.

The photocell circuit comprises a photocell of the barrier layer type parallel with a condenser and both are connected in series with a resistor, D. C. being supplied to the circuit through a rectiu ner irom a source of constant A. C. voltage. Current passed by the photocell is approximately proportional to the light intensity falling thereon and it discharges the parallel condenser at a rate proportional to this illumination. The condenser is recharged every 21/2 milliseconds to the pealr value of the constant voltage source and the now of charging current through the series resistor produces a voltage drop which is used as information.

The reference circuit duplicates the photocell circuit except for a fixed resistor in place of the photocell which is selected as to resistance value according to camera shutter speed and can comprise a gang of resistors selectively put in circuit to suit various shutter speeds. The voltage across the two charging resistors are compared and when they are equal no signal results but if one or the other voltage is predominant, the resultant signal will have a polarity in accordance with the predominating voltage.

After nltering, phase shifting and suitable ampliiication, this signal can be applied to one phase of a two phase motor, the other phase of which is associated with the source of initial A. C. supply.

Reference is now made to the accompanying drawings illustrating the present invention, in which:

Figure l is a block diagram of the automatic control system of the present invention showing its component parts;

Figure 2 is a schematic circuit diagram of the system of automatic control of the present invention; and

Figure 3 is a circuit diagram of lter and amplifier parts of the system of Fig. 2, showing certain modifications.

Referring specifically to Fig. 1 of the drawings, the operating principle of the system can be readily understood as applied to a stereographic aerial camera having twin lenses It. Each lens will have associated therewith the usual iris dia phragm ii for varying the eilective area or the lens from its maximum aperture position to a minimum position as is usual with such cameras and the two iris diaphragms for lenses i3 will be coupled together in a stereographic camera.

A suitable electric motor i2 is provided for causing rotation of the parts of the diaphragme .which affects the control of lens aperture.

This conventional lens assembly is modified in accordance with the present invention by mounting suitably associated with the lenses i a photocell i3 of the barrier-layer type also having an iris diaphragm lil which is coupled with the drive of motor i2 so that it moves in unison with diaphragms ii by means of suitable gearing shown dotted at l5. The modified lens assembly illus trated is suitably mounted on the aerial or other camera with which it is to be used and is so designed that the photocell I3 views the eld which is to be photographed by the stereo lens assembly IG.

Since the output of photocells of the type utilized may be affected by temperature changes of extreme range such as may be encountered in aerial flight a photocell heater i6 is mounted in thte lens assembly in position to maintain the photocell i3' at a constant temperature of desired value, this heater being turned olf and on by a thermostat il to maintain this constant temperature, a suitable source or" electric supply being supplied thereto in any usual manner.

The lens assembly forming part of the camera conventionally is carried by the aircraft in suitable position ior taking the desired photographs.

The control unit, to be now described, can however, occupy a position completely remote from the camera-lens assembly and need only be connected thereto by suitable multi-wire cable whereby the intelligence obtained from the photocell i3 can be received and whenever correction of lens apertures is required, this information can be transmitted back to the motor i2.

The control unit indicated in Fig. l consists of several components. A power supply V5 is fed from a suitable source of alternating current, such as the H5 volt 'it cycle available on many types of aircraft. One output of the power supply is fed by leads le to a device which will maintain and deliver a constant A. C. voltage irrespective of variations of current supplied to or from the power supply I3.

The constant voltage thus obtained in fed to a comparator device 2| by a lead 22. Comparator 2i receives the intelligence from the photocell I3 by a lead 23 and is so constructed, adapted, and arranged to operate that this intelligence is matched against a reference 2d communicating with the comparator through lead i5.

The values of reference 2li are preset to suit the average light conditions expected or encountered during the photographic run and these will vary and be varied to suit factors such as the emulsion speed of the film being used.

When these average light conditions are realized the reference circuit and the intelligence circuit will be in balance and no signal will result. When, however, the two circuits are out of balance a signal will result in magnitude and polarity equivalent to the change in light conditions above or below average observed.

This signal is nrst nltered to eliminate the high harmonics of the pulse-type signal and this filter 26 fed from the comparator by lead 2l is also adapted to rotate the phase angle by ninety degrecs. Filter 26 thus supplies a 4.90 cycle filtered signal shifted ninety degrees with respect to the line or reference voltage and its relative polarity thereto will depend upon whether the photocell i3 is receiving too much or too little light.

This signal is passed by lead 28 first to a drive amplier 29 and then by lead 3S to a power ampliner 3i, the power supply It feeding current to the power amplier by lead 32.

The amplified nltered and phase shifted signal is now applied by lead 33 to the one phase of the two phase motor i2, the other phase of which is supplied by the reference or line voltage.

Accordingly, motor l2 will rotate in one or the other direction under urge of this signal to modify the eiective apertures of the stereolenses i0 and photocell I3 until the intelligence circuit of comparator 2i is again in balance with the circuit of reference 24.

Thus, by this means any variation of light conwith the amount of light applied thereto.

I ditions yobserved by photocell I3 will cause irndamping or anti-hunt means are desirable and to this end. means are preferably provided to damp the movements of motor I2 with a derivative of the error signal detected by comparator 2 I.

To this end, also coupled to move with motor i2 by linkage 3:3 is a variable resistor 35 which is suitably energized by an independent source of power than that used for the other components. The voltage drop across resistor 35 will vary as function of lens aperture and Vthis is led through condenser C3 to comparator 2l by lead 35. rThe derivative thus obtained has a maximum value which corresponds to the actual maximum rate of change of area of the irises II and |11. The derivative is added to the input signal or intelligence from photocell I3'but it will not have appreciable eifect until the null point of motor i2 is practically approached and then it can come into effect as a relatively large retarding force to abruptly stop the mechanism.

Since the current passing through resistor 35 is a measure of the actual position of the iris i4 of photocell a suitable ammeter 3l may be connected in lead |02 and if this is calibrated in f stops it will give a fairly accurate indication ci the position of the irises of photocell and lenses and. thus can warn of changed light conditions necessitating resetting of the Values of the reference 24, to establish a new average illumination condition for a new shutter speed.

Referring to Fig. 2, there is shown a schematic representation which essentially can perform the functions of control hereinbefore described.

In Fig. 2, a iirst intelligence network A is associated with resistor R1 and a second reference network B is associated with resistor Network B forms reference 24 of Fig. l and comprises a condenser C2 in parallel with a lixed resistor R3, both fed from a source` of A. C. supply at a frequency of 400 cycles and at a very low operating voltage through a rectifier 38. Resistor R2 is in series with condenser C2 and resistor R3. Network A comprises a condenser C1 connected in parallel with both a xed resistor R4 and is fed by photocell I3, the source of A. C. supply being fed thereto through a rectifier 39.

When photocell I3 is illuminated, the current which passes thereby will be approximately proportional to the intensity of the light and the condenser Ci will be discharged at a rate depending upon the intensity of light and will be recharged in a fraction of a second by the rectified A. C. supply. A photocell of the barrier-layer type when arranged in a circuit as in Fig. 2, will act as a semi-conductor and its resistance will vary Thus, the amount of discharge of condenser C1 through photocell is is a measure of the light intensity.

The actual discharge of the condenser Ci will be over the major portion of the cycle and over the remaining portion, which is very small, the fluctuating direct current will charge the condenser.

lf the network B has the same characteristics as network A then at a predetermined light intensity on photo-cell I3 there will be no voltage drop between points y at ground potential and :c and the voltages across the resistors Ri and R2 'will be balanced. Upon change of light intensity either above or below the present and preselected level there will be an out-of-balance between these voltages, namely, a voltage differential beviris I4A associated with photocell I4 will increase or decrease in diameter by this ytween points :c andy, which will be proportional substantially'to the change of light intensity.

The signal thus produced is picked up at point :c and fed to the amplifier SI. After passing the *ltering and phase shifting lter 26 and amplication, the output is applied to the variable phase of the two-phase motor I2, the ilxed phase of which will be suitably Iexcited from the 40)- cycle source of supply. `Motor I2 will. thereby rotate under the influence cf the amplified signal, the magnitude and direction of rotation being determined by the increase or decrease of light intensity on photocell I3.

The rotation of motor I2 is applied to the The iris rotation in accordance with the increase or decrease of light until the intensity of illumination on the photocell'is established at the preselected, desired value. The system will then be again in balance with no voltage drop between points .t and y.

In the specific embodiment, the motor l! `adapted to simultaneously rotate the iris associated with the camera lenses IQ so that the light upon the film in the camera will remain at a constant value, which value can be preselected byproper choice of the values of reference 24. This constant value will bear a Vpositive relationship to the amount of light established as the desired value on the photocell I3 and on the f lm.

As a further refinement, to provide anti-hunt features and to anticipate the changes'to take place, the 'motor I2 also drives the arm 34 of a variable resistor R5' (35 in Fig. l), one end of which is connected through a condenser C3 with the photocell I3 at point e. Arm 34 is grounded and a xed resistor R5 is connected in series with the variable resistor and with a source of D. C. supply at a suitable voltage. Obviously, as arm 34 moves under urge of motor I2, the total resistance of R5 plus R5 will be altered and a voltage proportional to the rate of change of the light intensity on the photocell will be given to the networkl A. Since the magnitude of this voltage is proportional to the light intensity change, this will give advance information as to the movements of the iris vIll to the network A and also will act to prevent hunting.

The values of resistors R5 and Rs (Fig. 2) can obviously be chosen to provide any desired result in the damping or Vanti-hunt system.

Rs may be a non-linear potentiometer for example and by selecting the right resistance values for saine and for resistor R5 the amount since the absolute accuracy of the photocell I3 depends to a great extent on a constant applied voltage.

To this end, as shown in Fig. 2, a Wheatstone bridge circuit is provided which utilizes four resistors and a tungsten Vlament lamp.

A power supply transformer 40 has a primary winding l which is Vfed ironia source of supply such as 400 cycle 115 volt' alternatinurrent through a control switch 42, a pilot light 43 being in parallel with primary 4l tonindicate when current is on. One secondary windingli serves as input to the bridge 45 in the arms of which are two matched precision resistors 46, 41 having values of 270 megohms each, a precision-resistor t3 of 100 megohms and in the fourth arm is a tungsten lament lamp i9. Between the output points t of the bridge is a variable resistor 5l whereby initial balance of the bridge can be obtained.

The nlament of lamp 4Q will change its resistance with the applied voltage fluctuations and thus will unbalance the bridge as necessary to maintain an output from points k50 which is constant to a satisfactory degree.

Transformer 40 has other secondary windings for the purpose conventionally of providing power supply for the variouselectronic tubes used in amplifier 3 i To this end a high voltage winding 52 will deliver for example, V390 volts for the plate current of the various tubes to bedescribed whilst a low voltage secondary winding 53 supplies the heaters for the tubes at, for' example, 12 volts. A suitable grid bias supply such as volts is provided by a third secondary winding 5ft. Y

inasmuch as the connections between these electric supplies and the various tubes are conventional for purposes of clarity the specific wiring thereof is omitted.V

Fig. 3 Vis a circuit diagram of the system according to the present inventionvfor amplifying the out-oi-balance signal of comparator 2| and applying this to motor I2 for the purpose described. Y

Referring to Fig. 3, lead 55 comes from point m oi' Fig. 2 whereby the signal from comparator 2l may be rst ltered and then amplified.

The high harmonics of the pulse type signal are eliminated in lter 26 which comprises a single pi-section formed of choke 56 and condensers 51 and 5t with load resistors 59 and 60 and simultaneously this unit serves the purpose of rotating the phase angle by 90.

This voltage is applied to the grid 62 of a rst vacuum tube Si whose plate 63 has a lead 64 connected through a dropping resistor with the 300 volt plate supply. Cathode 61 of tube 6| is grounded by lead 68 to ground 69.

The amplified signal is fed from plate 63 to the grid 'it of a second vacuum tube 1| through a condenser 'i2 and grounded resistor 13. Re-

sistor 'F3 is preferably a Thyrite resistor in order to eliminate blocking and to give the amplier a rapid recovery time which even with a considerable overload of inputsignal may be as rapid as 15 milli-seconds. Y

The plate l@ of tube l is energized through a dropping resistor 'i5 from the 300 volt line by a lead 'it and the cathode 82 is grounded through resistor Sil and condenser 83 to ground wire E9.

Thus, the 400 cycle input signal from point ill be amplied in two stages toga degree usable in the driver stage of amplifier 29 (Figi), the polarity or the signal depending on whether photocell i3 is receiving too much or too little light. Tubes 5l and 'H may be separate highmu triodes or may be the two halves of a double triode vacuum tube.

The driver stage ofthe system comprises a triode vacuum tube 11 having its grid 18 coupled to the output of tube H by a condenser '19 and resistor 86, a blocking condenser 8l being used between lead 82a from plate kH3 and ground wire 69, to prevent feedback.

Cathode 35 is grounded to ground wire 89 through condenser 8? and resistor 8E and plate S8 is energized from the 390 volt supply through dropping resistor 89.

Resistor Si! is also preferably a rhyrite resistor in order to reduce recovery time and prevent blocking,

Tube 'Il is transformer coupled to the output sta-ge through a transformer 90, the primary winding Si of which is in the plate circuit of tube 11 by lead 92.V

Secondary winding of transformer' St feeds the grids of a double-triode vacuum tube Q3 and has a coupling condenser Sil thereacross. The two cathodes Q5 of theV double-triode are grounded to ground wire 69 and the two plates t a-re energized by the 390 volt supply through the primary winding 9i of the output transformer et by means of a center tap 9S and lead iit. The secondary winding lili is directly connected with one phase winding of motor i2, the other phase Winding oi which will be energized by 400 cycle supply as shown in Fig. 1.

Since the two voltages supplied are at phase angle to each other, motor i2 will rotate in accordance with the amplified signal, as previously described.

A suitable bias supply of, for example, 20 volts D. C. is applied as a xed grid bias to the grids of double-triode 93, whereby to obtain the maximum power output from the output stage.

As previously described it is desirable with a system of the type so far described to damp operation with a derivative of the error signal, whereby to prevent hunting and cause a retarding force at the end of the cycle of movement halting the mechanism.

Accordingly, a derivative of the displacement is taken which is proportional to the velocity of the whole servo-mechanism forming this system, including the mechanical inertia.

To this end, a variable resistor 35 is provided, the moving arm H32 of which is mechanically linked to the iris-motor drive (36 in Fig. 1). By this means the voltage drop across the resistor changes as a function of iris opening.

In order to obtain accurate results it is necessary to have the current through resistol1 3Q independent of variations of voltage or the like, to the rest of the system. To provide this voltage and keep it regulated a triode vacuum tube |03 is provided having its plate Hit energized from the 300 volt supply by a lead H35. The grid |96 of tube l is connected to a neon bulb iii? by a lead IGS the other side of which is grounded to ground wire te. Neon bulb itl is excited from the 300 volt supply through lead l i l and a dropping resistor l l The cathode H2 of tube m3 is connected through two matched series resistors i I3 and i lli with one side of variable resistor 35 and a filter condenser H5 is connected between ground wire 69 and the common connection o resistors H3 and lili to point H5. From this point, connections a-re made to point e of Fig. 2 in order that the derivative may be introduced.

Tubes HB3 and 'il may be separate triodes or be the two halves of a double triode vacuum tube. The iris position indicator 3l will be suitably connected in circuit between ground wire 59 and lead |02. l

By means of the system hereinbefore described large changes of illumination can be automatically compensated for and the minimum-to-maximum illumination ratio may be as high as any iris can be constructed.

Obviously, by changing the value of resistor R3 (Fig. 2) of the reference information, a new point of balance will be created.

` accordance with aI further feature of the present invention there is provided a plurality of resistorsR'a with a selector switch to place a de- I'sired value into position. By calibration, such resistors c an be selected for Idifferent shutter fspedsand can be operated in connection with a idi'al marked for shutter speeds.

As shown in Fig. 2, a selector |20 cooperates with a resistor or gang of resistors R7 to alter the valuesY of network B and set up a new balance point between circuits A and B.

Furthermore, by varying the aperture f photocell I3 independently of the irises Il and ld different standards of light intensity can be set up.

In the present invention one of several perforated baffles may be inserted in front of the photocell by means of a selector knob and desirably the knob positions will be calibrated with ASA film speed ratings.

While hereinbefore reference has been made to an iris for modifying the effective area of the photocell inversely until the point of balance is reached other known means can replace the iris such as a pair of opposed density wedges which fare 'moved by motor I2 in front of the photocell opposite directions, whereby the same result is achieved as by opening or closing the iris and the present invention contemplates any such other means as equivalents of the iris I4, within the spirit and scope of the appended claims.

Iclaim:

1. A system for the automatic control of the lens aperture ofan aerial camera to maintain the light intensity at they film in the camera at a preselected value including a barrier layer photocell arranged to view the same scene as the camera lens; a lens iris and a photocell iris; a reversible two-phase electric motor drivingly connected with said irises; an electric heater associated with the photocell and a thermostat controlling said heater to the photocell as a substantially constant temperature; anda control unit `for operating said motor in one or the other direction to close theV irises comprising a photocell circuit having a condenser in parallel with said photocell, a resistor in series with one side of the photocell and condenser and a rectifier in series with the other side of said photocell and condenser; a reference circuit having a condenser and rst resistor in parallel; a second resistor in series with one side of the condenser and resistor and a rectifier in series with the other side of the condenser and rst resistor; a source of alternating current supplies at a constant voltage connected to said rectiers, the condensers of the two circuits being matched and the resistor of the photocell and second resistor of the reference circuit being matched; the value of said first resistor in the reference circuit establishing Xed reference information against which variable information cause by photocell signal under changes of light intensity can be matched; a lter connected to receive error signals when said circuits are unbalanced by such changes in light intensity; means to shift the phase angle of the error signal 90, a driver amplifier connected to receive the filtered signal; a power amplier receiving the signal from the driver amplier and connections from said power amplifier to said motor, to rotate same in accordance with the error signal until the photocell iris is opened or closed to an extent to cause balancing of the circuits again.

2. A system for the automatic control of the lens aperture of an aerial camera to maintain the light intensity at the lzn in the camera at a preselected value including a barrier layer photocell arranged to View the same scene as the camera lens; va lens iris and a photocell iris; a reversible two-phase electric motor drivingly connected with said irises; an electric heater associated with the photooell and a thermostat controlling sa-idheatcr tomaintain the photocell at a substantially constant temperature; and a control unit for operating said motor in one or the other direction to open or close the irises comprising a photocell circuit having a condenser in parallel with said. photocell, a resistor in series with one side of the photocell and condenser and a rectifier in. series with the other side of said photocell and condenser; a reference circuit having a condenser and rst resistor in parallel, a second resistor in series with one side of the condenser and resistor and a rectifier in series with the other side of the condenser and nrst resistor; a source of alternating current supplies connected to said rectifiers, the condensers of the two circuits being matched and the resistor of the photocell and second resistor of the reference circuit being matched; the value or" said rst resistor in the reference circuit establishing fixed reference information against which variable information caused by photocell signal under changes of light intensity can be matched; a filter connected to receive error signals when said circuits are unbalanced by such changes in light intensity; a power amplifier receiving the signal and connections from said power amplie to said motor, to-rotate same in accordance with the error signal until the photocell iris is opened or closed to an extent to cause balancing of the circuits again.

3. A system for the automatic control of the lens aperture of an aerial camera to continuously maintain the light intensity at the film in the camera constant at a preselected value including a photocell arranged to View the same scene as the cameralens; a lens iris and a photocell iris; a reversible electric motor drivingly connected with said irises; and a control unit for operating said motor in one or the other direction to open or close the irises comprising a photo-cell circuit having a condenser-in parallel with said photocell, a resistor in series with one side of the photocell and condenser and a rectier in series with the other side of said photocell and condenser; a reference circuit having a condenser and rst resistor in parallel, a second resistor in series with one side of the condenser and resistor and a rectifier in series with the other side of the condenser and rst resistor; a source of alternating current supplies at a constant voltage connected to said rectiers, the condensers of the two circuits being matched and the resistor of the photocell and second resistor of the reference circuit being matched; the value of said rst resistor in the reference circuit establishing xed reference information against which variable information caused by photocell signal under changes of light intensity can be matched; a filter connected to continuously receive error signals when said circuits are unbalanced by such changes in light intensity; means to shift the phase angle of the error signal 90, a driver amplifier connected to receive the filtered signal; a power amplier receiving the signal from the driver amplier and connections from said power amplifier to said motor, to rotate same in accordance with the error signal until the photocell iris is' opened or closed to an extent to cause balancing of the circuits again and reestablishment of the light intensity at the lm of the preselected value.

4. A system for the automatic control of the lens aperture of an aerial camera to continuously maintain the light intensity at the lm in the camera constant at a preselected value including a photocell arranged to view the same scene as the camera lens; a lens iris and a photocell iris; a reversible electric motor drivingly connected with said irises; and a control unit for operating said motor in one or the other direction to open or close the irises comprising a photocell circuit having a condenser in parallel with said photocell, a resistor in series with one side ofthe photocell and condenser and a rectier in series with the other side of said photocell and condenser; a reference circuit having a condenser and first resistor in parallel, a second resistor in series with one 'side of the condenser and resistor and a rectifier in series with the other side of the condenser and first resistor; a source of alternating current supplies connected to said rectiers, the condensers of the two circuits being matched and the resistor of the photocell and second resistor of the reference circuit being matched; the value of said first resistor in the reference circuit establishing xed reference information against which variable information caused by photocell signal under changes of light intensity can be matched; a filter connected to continuously receive error signals when said circuits are unbalanced by such changes in light intensity; a power ampli-er receiving the signal and connections from said power amplifier to said motor, to rotate same in accordance with the error signal until the photocell iris is opened or closed to an extent to cause balancing of the circuits again and reestablishment of the light intensity at the nlm of the preselected value.

5. An automatic light control unit for use in maintaining a preselected constant light intensity under varying conditions of external illumination including in combination a barrier layer photocell, a reference voltage source in circuit with said photocell and opposing said photocell whereby zero potential may be established at a point between the photocell and voltage source, a voltage source including a variable resistor controlled in accordance with the setting of an irisI diaphragm controlling the light illuminating the photocell, a comparator to derive current alternately from the reference voltage source and photocell or through a capacitor from the second voltage source, a reversible motor controlling the opening of the iris diaphragm and variable resistor, means for supplying pov/er selectively to drive the motor in one direction or the other in accordance with the unbalance at the comparator until the voltages on said comparator are Zero, and a lens iris diaphragm moved with the first iris diaphragm, whereby constant intensity illumination is supplied through said lens to said photocell.

6. A light control unit as claimed in claim 5 in which the second lens iris diaphragm controls the light passed by a camera lens to the sensitized surface in a camera.

7. A light control unit as claimed in claim 5 in which the motor is a two phase motor excited by alternating current from a constant source of power and by current from said comparator varying with the light intensity as measured by said photocell.

8. A light control unit as claimed in claim 5 in which the motor is a two phase motor excited by alternating current from a constant source of power and by current from an amplifier controlled from the comparator by light intensity as measured by the photocell.

9. A light control unit as claimed in ciaim 8 in which the amplifier is controlled in part by a filtered signal current displaced in phase from the light-intensity controlled current.

10. A light control unit as claimed in claim 5 in which a thermostaticaily controlled heater is provided for the photocell.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 1,919,182 Fitzgerald July i8, 1933 2,100,755 Shepard Nov. 30, 1937 2,156,440 Veber May 2, 1939 2,297,262 Tonnier Sept. 29, 1942 2,325,853 Harrison Aug. 17, 1943 2,412,424 Rath Dec. 10, 1946 2,417,506 Lamb Mar. 18, 1947 2,420,058 Sweet May 6, 1947 2,421,476 Belar et al. June 3, 1947 2,434,101 Cann Jan. 6, 1948 2,453,693 Armstrong et al. Nov. i5, 1948 

